There is growing evidence that pet-assisted therapies may be beneficial for children with physical, emotional, and developmental disorders. In addition, human-animal interaction (HAI) and pet ownership are associated with lower stress and increased prosocial behavior and psychological well-being in children and adults. However, a number of research questions remain unanswered. First, do biological mechanisms (e.g., oxytocin) associated with HAI in previous studies mediate relationships between HAI and prosocial behavior and stress? Second, do the positive benefits of pet ownership during childhood have lasting effects on biology and behavior in late adolescence and young adulthood? Third, do individual or group differences, including both person and context characteristics, as well as experimentally-induced stress, moderate the effects of HAI on biology and behavior? The proposed study will utilize a sample of 96 individuals aged 18-25 to specifically address the above research questions. Subjects will be brought to the PI's laboratory for a brief (20 minute) interaction with a friendly, unfamiliar dog (experimental HAI). Indices of prosocial behavior and stress will be obtained via laboratory paradigms. Collection of repeated plasma oxytocin and salivary cortisol samples and measures of heart rate variability will allow us to assess the impact of experimental HAI on underlying biological mechanisms. Half of study participants will have a history of childhood dog ownership, and the other half will not have previously owned pets. No subjects will currently live with pets. Half of the study participants will undergo HAI prior to the stress paradigm, and the other half will interact with te dog after the stress paradigm, allowing us to determine whether HAI reduces biological markers of stress, and whether stress facilitates the release of oxytocin during HAI. Moreover, we will collect self-report and clinical interview data from all 96 subjects on current behavioral and emotional outcomes (e.g., aggression, depression, prosocial behavior), person characteristics (e.g., empathy, attitudes towards animals), and measures of social environment (e.g., recent stressful life events, social support, family characteristics) that may moderate or predict the effects of HAI on biology and behavior. Finally, we will assess the timing of exposure to dogs, intensity of attachment to pets, and dog characteristics as factors that may predict individual differences in biology and behavior among prior dog-owners.